BEIRUT — Rebels fighting in Syria's civil war crossed into Lebanon and raided a border town Saturday, killing and capturing security force members in the most serious incursion into the tiny country during its neighbor's 3-year-old conflict.

The rebels, who included foreign fighters, demanded to trade soldiers and police officers it captured in Arsal for some of the "most dangerous detainees," the Lebanese army said in a statement. Masked gunmen roamed the streets as Lebanese helicopter gunships flew over the town, about 90 kilometers (55 miles) from the capital, Beirut.